The invention consists in a differentiated sandwich laminate (comprising an outer layer, an intermediate layer or xe2x80x9ccentrexe2x80x9d, and an inner layer), in which the outer layer is made using a type of resin that is different from, and incompatible with, the resin used for making the inner layer.
The inner layer and outer layer of sandwich laminate or the adjacent layers of two different sandwich laminates, made according to the invention, are made to adhere together in a watertight manner using mechanical-assembly means (bolts, rivets, etc.).
Composite materials, i.e., materials consisting of a resin (or other equivalent material) in which are embedded fibres of a chemically inert material (e.g., glass fibre), preferably in the form of one-directional layers and/or cloths made using the said material, are well known in the state of the art and are widely used for making (relatively) light structural elements presenting high characteristics of resistance. One of the fields in which composite materials have been used most successfully (principally, but not exclusively, in the nautical sector) is in the production of the so-called  less than  less than sandwich laminates greater than  greater than , which are characterized by a pair of layers of composite material (also known in the art of the sector as  less than  less than skins greater than  greater than ), which are thin and strong and are applied on the opposite faces of a (somewhat thick) central body called  less than  less than centre greater than  greater than ; the  less than  less than skins greater than  greater than  are fixed to the central body by means of an adhesive material designed to transmit the loads applied to the  less than  less than skins greater than  greater than  onto the central body.
The  less than  less than skins greater than  greater than  are in turn obtained by rolling, i.e., by superimposing and getting to adhere together a number of elementary layers of a composite material consisting of a supporting fibrous material embedded in a matrix made of resin (or other equivalent material).
The methods used (or that may be used) for making a sandwich laminate will not be described herein because they are in themselves known and are in any case extraneous to the present invention.
Sandwich laminates are advantageous in that they present excellent mechanical characteristics with a contained weight. This advantageous feature is, however, normally obtained at the expense of a greater constructional complexity with respect, for example, to a simple laminate, and at the expense of resistance to delamination (i.e., to the detachment of at least one  less than  less than skin greater than  greater than  from the central body), which may be critical and which depends upon the materials used, upon the care taken and upon the production process with which the sandwich laminate is made, as well as upon the type of stresses which the sandwich laminate undergoes (or may undergo) during use.
The resins normally used for making the  less than  less than skins greater than  greater than  of a sandwich laminate are polyester, vinyl ester or epoxy resins. Each one of these types of resins has qualities and defects which condition the possibility of them being used, in particular, in the boat-building field.
Polyester and vinyl ester are suited for being used for making the outer surface of a hull in that they afford, respectively, good impermeability and good resistance to osmosis. However, their mechanical and technological characteristics are modest; these resins are in fact brittle, and it is necessary to alternate cloth layers and layers of a filling material (known in the art of the branch as  less than  less than mat greater than  greater than ) which has poor mechanical characteristics and absorbs large amounts of resin.
Epoxy resins have better mechanical and technological characteristics (they do not require the use of  less than  less than mats greater than  greater than ) but are not suited for use in making the outer surface of a hull in that they do not possess good impermeability or resistance to osmosis; moreover they are not available on the market in the form of a waterproofing material, known in the state of the art as xe2x80x9cgel coatxe2x80x9d, there do not exist  less than  less than mats greater than  greater than  that are compatible with epoxy resins, and epoxy resins cannot be associated to a polyester waterproofing gel coat because it is (practically) impossible to get an epoxy resin to adhere to a polyester or vinyl ester.
Both  less than  less than skins greater than  greater than  of the sandwich laminates currently known and normally used (in particular in the boat-building field) are made using the same type of resin (polyester/vinyl ester or epoxy resins). In fact, it is common experience among technicians operating in the sector of sandwich laminates that, if the  less than  less than skins greater than  greater than  are made with resins of different types, it is very difficult (if not practically impossible) to get two constructional elements made with sandwich laminates (such as the hull and deck of a boat) to adhere together in a watertight way and/or to seal the  less than  less than skins greater than  greater than  of a sandwich laminate together.
To overcome this limitation, there are currently on sale materials (known in the art as  less than  less than primers greater than  greater than ) which should favour adhesion between epoxy resins and polyester/vinyl ester resins, but the results obtainable using these materials are altogether unsatisfactory and unreliable, as has been experimentally ascertained by the applicant.
Making both of the  less than  less than skins greater than  greater than  of a sandwich laminate using the same type of resin (polyester/vinyl ester resin or epoxy resin) means, however, foregoing an adequate exploitation of the advantageous characteristics presented by each of the two types of resins, i.e., foregoing the numerous advantages (of a technical/constructional, economic nature, etc.) which may be achieved by making each  less than  less than skin greater than  greater than  of a sandwich laminate with the type of resin (polyester/vinyl ester resin or epoxy resin) which presents the characteristics best suited to the environment with which the said  less than  less than skin greater than  greater than  is to come into contact.
Long research studies and repeated experimental tests have enabled the applicant to overcome the limits and drawbacks described above by making a differentiated sandwich laminate, the  less than  less than skins greater than  greater than  of which are made using different resins (in particular a polyester/vinyl ester resin and an epoxy resin) and connecting together the two sandwich laminates made according to the invention, and/or the  less than  less than skins greater than  greater than  of the same sandwich laminate, using mechanical-assembly means.
The subject of the present invention is a differentiated sandwich laminate comprising a pair of layers of composite material applied to the opposite faces of a central body, in which the resin used to make one of the aforesaid layers is different from the one used to make the other layer and is incompatible with it.
Preferably one of the layers of composite material is made with a polyester/vinyl ester resin, whilst the other is made with an epoxy resin.
The layers of composite material of a sandwich laminate, or else of two or more adjacent sandwich laminates, are connected together in a watertight way using mechanical-assembly means.